(1) Field of the Invention
This invention relates to a printed board or printed wiring board and, more particularly, to a printed wiring of a printed board provided with a plurality of through holes located on intersecting points (hereinafter, designated as "grid points") of an X-Y orthogonal basic grid.
(2) Description of the Prior Art
With regard to a printed board, and in particular, a multilayer printed board on which a high density of wiring is required, various standardizations have been used for facilitating the design of a conductor pattern and the manufacture of the printed board. For example, through holes are located on the grid points at a standardized pitch, and the conductor pattern is formed so that the conductors pass between these grid points. For ensuring a high density of wiring under such circumstances, various kinds of conductor patterns have been proposed.
Some typical known conductor patterns will be described below, with reference to FIGS. 1 through 3 of the accompanying drawings, in order to ensure a good understanding of the background of the present invention.
Referring to FIG. 1, an orthogonal conductor pattern is illustrated in which through holes H.sub.11, H.sub.12, H.sub.21 and H.sub.22 are located on the grid points of an X-Y orthogonal basic grid G. Two conductors PX1 and PX2 in a conductor layer extend in the X direction, and two conductors PY1 and PY2 in another layer extend in the Y direction. Wiring between two desired points is provided by a combination of straight conductors in the X and Y directions. At a turning point of the wiring, the conductors in the different layers are connected via the through holes.
The above described orthogonal conductor pattern is advantageous in that it is simple to design and manufacture, but it is not completely satisfactory for a printed board in which a high density of wiring and high-speed signal transmission are required. For example, in a case wherein two points of a printed board which are diagonal to each other are connected in accordance with the orthogonal conductor pattern, a wiring length between these points is .sqroot.2 times, at the maximum, of the straight diagonal distance, thereby causing some problems, such as a lag in signal transmission and a voltage drop. In addition, the increased wiring length results in a longer length of the entire conductor than required, thereby not achieving a high density of wiring.
To avoid the above mentioned problems, the use of an oblique straight conductor pattern has been proposed. Referring to FIG. 2, a typical known oblique pattern is illustrated in which conductors PO.sub.1, PO.sub.2, PO.sub.3 and PO.sub.4 extend obliquely at an angle of 45.degree., with two conductors passing between adjacent grid points arranged in the X direction and, also, between adjacent grid points arranged in the Y direction. It will be understood that the oblique conductor pattern layer, or the oblique pattern layer in combination with X-Y orthogonal conductor pattern layers, ensures a reduced wiring length compared with using only the orthogonal conductor pattern. However, as compared with the orthogonal conductor pattern illustrated in FIG. 1, the conductor spacing of the oblique pattern, i.e. the distance between two adjacent conductors, and also, the distance between a conductor and an adjacent through hole land are smaller, even though the conductor width may be made smaller. This has operational advantages. It will also be understood that the area between the conductors PO.sub.2 and PO.sub.3, and between the through holes H.sub.11 and H.sub.22, is not effectively used.
An improved conductor pattern for obviating these defects in the 45.degree. oblique straight conductor pattern is illustrated in FIG. 3, in which conductors PO.sub.1 ', PO.sub.2 ', PO.sub.3 ' and PO.sub.4 ' extend in the form of zigzag lines without contacting the through holes. As a result, in comparison with the conductor pattern shown in FIG. 2, the distribution of the conductors is more uniform and there is substantially no area of non-use. However, the degree of meandering of the conductor is generally large, thereby resulting in an increase in the wiring length and also an increase in the parasitic inductance. Furthermore, the large degree of meandering of the conductor results in the design of the conductor pattern being complex and a requirement for a large size drawing machine, particularly in the case of automatic drawing by means of an electronic computer, due to a large number of turning points, and an increase in drawing time.
Furthermore, in the patterns shown in FIGS. 2 and 3, two conductors pass between adjacent through holes arranged in the X direction and in the Y direction, while four conductors pass between the through holes H.sub.12 and H.sub.21. This type of congestion of the conductor pattern can not be obviated in so far as the 45.degree. oblique straight conductor pattern is concerned.
Regarding the design of the conductor pattern, there is a further problem, in that the number of conductors which can pass between the through holes at a predetermined distance depends on the width of the conductors, the conductor spacing, and the sizes of the through holes and of through hole lands. If the width of the conductors and/or the size of the through hole lands are made smaller, it is possible to ensure an increased number of conductors, as well as a required conductor spacing. However, a reduction in the width of the conductors and/or a reduction in size of the through hole lands are necessarily limited. In particular, if the through hole lands are reduced in size, a significant harmful effect may occur in a case wherein an eccentricity of the through holes with respect to the through hole lands exists.